1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and more particularly to an electrophotographic photosensitive member used in an electrostatic copying machine, a laser beam printer and the like.
2. Description of the Related Art
The so-called Carlson's process has been widely used in order to obtain copied images using an electrophotographic photosensitive member. The Carlson's process includes a charging step wherein the photosensitive member is uniformly sensitized with corona charge, an exposure step wherein the sensitized photosensitive member is subjected to exposure of an original image to form a latent image corresponding to the original image on the surface of the photosensitive member, a development step wherein the latent image is developed with a toner developer to form a toner image, a transfer step wherein the toner image is transferred onto a transfer sheet such as paper, a fixing step wherein the transferred toner image on the transfer sheet is fixed, and a cleaning step wherein the residual toner on the photosensitive member is removed.
In order to obtain a high quality copied image in the Carlson's process, it is required for the electrophotographic photosensitive member to excel in both charging characteristics and photosensitive characteristics and to be low in residual potential after exposure.
As the electrophotographic material which satisfies these requirements, there are known inorganic photoconductors of selenium, cadmium sulfide, etc. However, as for such inorganic photoconductors, there are disadvantages in that they are noxious and are of a relatively high production cost.
In view of this, instead of those noxious materials, the so-called organic electrophotographic photosensitive members prepared using organic materials have been proposed. Such an organic electrophotographic photosensitive member excels in both processability and economy, and the function thereof can be freely designed. The conventional organic electrophotographic photosensitive member has a photosensitive layer comprising a charge generating material capable of generating electric charge upon exposure and a charge transporting material capable of transporting the generated electric charge.
In order for the organic electrophotographic photosensitive member to satisfy the desired various conditions, it is necessary to appropriately select the charge generating material and the charge transporting material.
A number of various compounds have conventionally been proposed as a charge generating material including phthalocyanine compounds, perylene compounds, quinacridone compounds, anthanthrone compounds and azo compounds. Examples of the known azo compounds are disclosed in Japanese Laid-Open Patent Publication No. 47-37543 and U.S. Pat. No. 4,507,471.
However, the conventional charge generating materials have disadvantages such as a small sensitive wavelength region and insufficient light stability. As a result, the photosensitive member comprising such a charge generating material has a low sensitivity and a poor durability.